A British Thermal Unit, or BTU, is the amount of heat energy required to raise the temperature of 1lb. of water a single degree Fahrenheit. The BTU is the standard unit of energy in the United States.
The BTU/h of an HVAC unit is calculated using the formula:
BTU/h = 4.5[standard air constant] x aiflow[cubic feet per minute - cfm] x enthalpy[change in heat]
1 BTU equals:
100,000 BTU's = 1 therm